1. Field of the Invention
This invention relates to the production of homo-, co- and ter-polymers of substituted styrenes such as p-acetoxystyrene (ASM), and/or alkyl acrylates and/or other monomers that are useful in photoresists and optical applications.
2. Description of the Prior Art
There is a desire in the industry for higher circuit density in microelectronic devices that are made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolutions. The use of shorter wavelength radiation (e.g., deep UV e.g. 190 to 315 am) than the currently employed mid-UV spectral range (e.g. 350 am to 450 am) offers the potential for higher resolution. However, with deep UV radiation, fewer photons are transferred for the same energy dose and higher exposure doses are required to achieve the same desired photochemical response. Further, current lithographic tools have greatly attenuated output in the deep UV spectral region.
In order to improve sensitivity, several acid catalyzed chemically amplified resist compositions have been developed such as those disclosed in U.S. Pat. No. 4,491,628 (Jan. 1, 1985) and Nalamasu et al, xe2x80x9cAn Overview of Resist Processing for Deep UV Lithographyxe2x80x9d, J. Photopolymer Sci. Technol. 4, 299 (1991). The resist compositions generally comprise a photosensitive acid generator and an acid sensitive polymer. The polymer has acid sensitive side chain (pendant) groups that are bonded to the polymer backbone and are reactive towards a proton. Upon imagewise exposure to radiation, the photoacid generator produces a proton. The resist film is heated and, the proton causes catalytic cleavage of the pendant group from the polymer backbone. The proton is not consumed in the cleavage reaction and catalyzes additional cleavage reactions thereby chemically amplifying the photochemical response of the resist. The cleaved polymer is soluble in polar developers such as alcohol and aqueous base while the unexposed polymer is soluble in non-polar organic solvents such as anisole. Thus the resist can produce positive or negative images of the mask depending of the selection of the developer solvent. Although chemically amplified resist compositions generally have suitable lithographic sensitivity, in certain applications, their performance can be improved by (i) increasing their thermal stability in terms of thermal decomposition and plastic flow and (ii) increasing their stability in the presence of airborne chemical contaminants. For example, in some semiconductor manufacturing processes, post image development temperatures (e.g. etching, implantation etc.) can reach 200xc2x0 C. Brunsvold et al., U.S. Pat. Nos. 4,939,070 (issued Jul. 3, 1990) and U.S. Pat. No. 4,931,379 (issued Jun. 5, 1990) disclose chemically amplified, acid sensitive resist compositions having increased thermal stability in the post image development stage. Brunsvold""s resist compositions form a hydrogen bonding network after cleavage of the acid sensitive side chain group to increase the thermal stability of the polymer. Brunsvold avoids hydrogen-bonding moieties prior to the cleavage reaction because such hydrogen bonding is known to unacceptably destabilize the acid sensitive side chain. Although Brunsvold resists have suitable thermal stability, they also have lower sensitivity and therefore are unsuitable in certain applications.
With respect to chemical contamination, MacDonald et al. SPIE 14662. (1991) reported that due to the catalytic nature of the imaging mechanisms, chemically amplified resist systems are sensitive toward minute amounts of airborne chemical contaminants such as basic organic substances. These substances degrade the resulting developed image in the film and cause a loss of the linewidth control of the developed image. This problem is exaggerated in a manufacturing process where there is an extended and variable period of time between applying the film to the substrate and development of the image. In order to protect the resist from such airborne contaminants, the air surrounding the coated film is carefully filtered to remove such substances. Alternatively, the resist film is overcoated with a protective polymer layer. However, these are cumbersome processes.
Therefore, there was a need in the art for an acid sensitive, chemically amplified photoresist composition having high thermal stability and stability in the presence of airborne chemical contaminants for use in semiconductor manufacturing. Apparently, this was accomplished in the invention outlined in U.S. Pat. No. 5,625,020 which relates to a photosensitive resist composition comprising (i) a photosensitive acid generator and (ii) a polymer comprising hydroxystyrene and acrylate, methacrylate or a mixture of acrylate and methacrylate. The resist has high lithographic sensitivity and high thermal stability. The resist also exhibits surprising stability in the presence of airborne chemical contaminants. However, one of the problems with this composition was that the process of preparing the polymer as outlined in column 3, lines 10-30 and in Example 1 (of U.S. Pat. No. 5,625,020) results in poor conversion rates and chemical cleavage of some groups in the repeat units. Thus, one of the objects of the present invention is an improved process for preparing the polymers used in the photoresist compositions.
The processes of the present invention provide methods which are fast, clean, anhydrous, and render the analysis of catalyst used therein in an easy manner. Furthermore, the polymer in solution, if desired can be further treated to provide a photoresist composition which can be directly used without isolating the polymer beforehand.
Prior Art
The following references are disclosed as general background information.
1. U.S. Pat. No. 4,898,916 discloses a process for the preparation of poly(vinylphenol) from poly(acetoxystyrene by acid catalyzed transesterification.
2. U.S. Pat. No. 5,239,015 discloses a process for preparing low optical density polymers and co-polymers for photoresists and optical applications.
3. U.S. Pat. No. 5,625,007 discloses a process for making low optical polymers and co-polymers for photoresists and optical applications.
4. U.S. Pat. No. 5,625,020 discloses a process for making a photoresist composition containing a photosensitive acid generator and a polymer comprising the reaction product of hydroxystyrene with acrylate, methacrylate or a mixture of acrylate and methacrylate.
5. EP 0 813113 A1, Barclay, discloses an aqueous transesterification to deprotect the protected polymer.
6. WO 94 14858 A discloses polymerizing hydroxystyrene without the protecting group.
Other patents of interest are U.S. Pat. Nos. 4,679,843; 4,822,862; 4,912,173; 4,962,147, 5,087,772; and 5,304,610.
All of the references described herein are incorporated herein by reference in their entirety.
This invention relates to a novel, one-pot, cost efficient process for the preparation of homopolymers and copolymers such as terpolymers and tetrapolymers of p-hydroxystyrene or substituted p-hydroxystyrene and/or alkyl acrylates and/or other monomers. The process involves polymerization of esters of p-hydroxystyrene (or its substituted analogs), alkyl acrylate monomers and/or one or more of ethylenically unsaturated monomers in an alcohol solvent in the presence of a free radical initiator. The anhydrous reaction mixture containing the so formed polymer is then subjected to transesterification conditions using a catalytic amounts of catalyst to result in co- and/or terpolymers of p-hydroxystyrene without cleavage of the alkyl ester in the acrylate repeat unit. Preferred embodiments include homopolymers of p-hydroxystyrene; copolymers of p-hydroxystyrene, and tert-butyl acrylate; and terpolymer of p-hydroxystyrene, tert-butyl acrylate and styrene. These polymers have a wide variety of applications including as photoresists in microelectronics industry.
The present invention thus provides, in part, a novel process for producing polymers that are used in photoresist compositions. The process is an improvement over the prior art and is quite efficient. Specifically, this invention provides a process for the preparation of a polymer of I, 
an acrylate monomer having the formula II, 
and/or one or more ethylenically unsaturated copolymerizable monomers (EUCM) selected from the group consisting of styrene, 4-methylstyrene, styrene alkoxide wherein the alkyl portion is C1-C5 straight or branch chain maleic anhydride, dialkyl maleate, dialkyl fumarate and vinyl chloride, wherein alkyl is having 1 to 4 carbon atoms, comprising the steps of:
a) subjecting a monomer of formula III, 
wherein R is either xe2x80x94OC(O)R5 or xe2x80x94OR5;
said monomer II, and/or one or more of said copolymerizable monomers to suitable polymerization conditions in a carboxylic alcohol solvent and in the presence of a free radical initiator at suitable temperature for a sufficient period of time to produce a polymer of corresponding composition;
b) subjecting said polymer from step a) to transesterification conditions in said alcohol solvent in the presence of a catalyst at suitable temperature such that the transesterified by-product ester formed is continuously removed from the reaction mixture to form the polymer of I, II, and said copolymerizable monomer;
c) contacting said polymer solution in said carboxylic alcohol solvent from step b) with a cation-exchange resin to remove said catalyst; and (optionally)
d) isolating said polymer from step c) either by precipitation from said alcohol solvent or by removal of said alcohol solvent by evaporation;
wherein:
i) R1 and R2 are the same or different and are independently selected from the group consisting of:
hydrogen;
fluorine, chlorine or bromine;
alkyl or fluoroalkyl group having the formula CnHxFy where n is an integer from 1 to 4, x and y are integers from 0 to 2n+1, and the sum of x and y is 2n+1; and
phenyl or tolyl;
ii) R3 is selected from the group consisting of:
hydrogen; and
methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl or tert.-butyl;
iii) R4 is selected from the group consisting of methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, tert.-butyl, tert.-amyl, benzyl, cyclohexyl, 9-anthracenyl, 2-hydroxyethyl, cinnamyl, adamantyl, methyl or ethyl adamantly, isobornyl, 2-ethoxyethyl, n-heptyl, n-hexyl, 2-hydroxypropyl, 2-ethylbutyl, 2-methoxypropyl, 2-(2-methoxyethoxyl), 2-phenylethyl, phenyl, and the like.
iv) R5 is C1-C5 alkyl, either straight or branch chain.
It is also within the scope of the present invention to prepare a homopolymer of formula I from the monomer of formula III. As one preferred embodiment, polyhydroxystyrene (PHS) can be prepared from acetoxystyrene monomer (ASM) according to the novel processes set forth herein.
The scope of the present invention thus covers, without limitation, (a) a homopolymer of formula I derived from formula III monomer; (b) a copolymer derived from formula II and formula III monomers; (c) a copolymer derived from formula III monomers and the EUCM; and (d) a terpolymer derived from monomers of formula II, formula III and EUCM.
In conjunction with formula II (an acrylate monomer) set forth herein, some preferred acrylate monomers are (1) MAAxe2x80x94methyl adamantyl acrylate, (2) MAMAxe2x80x94methyl adamantyl methacrylate, (3) EAAxe2x80x94ethyl adamantylyacrylate, (4) EAMAxe2x80x94ethyl adamantyl methacrylate, (5) ETCDAxe2x80x94ethyl tricyclodecanyl acrylate, (6) ETCDMAxe2x80x94ethyl tricyclodecanyl methacrylate, (7) PAMAxe2x80x94propyl adamantyl methacrylate, (8) MBAMAxe2x80x94methoxybutyl adamantyl methacrylate, (9) MBAAxe2x80x94methoxybutyl adamantyl acrylate, (10) isobornylacrylate, (11) isobornylmethacrylate, (12) cyclohexylacrylate, and (12) cyclohexylmethacrylate.
Copolymers, including ter- and tetra-polymers, having polyhydroxystyrene (PHS) and one of the above acrylate monomers are some of the materials that are made by the novel processes of the present invention.
In a preferred embodiment the reaction mixture may also comprise a second solvent. The second solvent is selected from the group consisting of tetrahydrofuran, methyl ethyl ketone, acetone, and 1,4-dioxane.
The carboxylic alcohol solvent is an alcohol having 1 to 4 carbon atoms and is selected from the group consisting of methanol, ethanol, isopropanol, tert.-butanol, and combinations thereof. The amount of solvent and/or second solvent used is not critical and can be any amount which accomplishes the desired end result.
The free radical initiator may be any initiator that achieves the desired end result. The initiator may be selected from the group consisting of 2,2xe2x80x2-azobis(2,4-dimethylpentanenitrile), 2,2xe2x80x2-azobis(2-methylpropanenitrile), 2,2xe2x80x2-azobis(2-methylbutanenitrile), 1,1xe2x80x2-azobis(cyclohexanecarbonitrile), t-butyl peroxy-2-ethylhexanoate, t-butyl peroxypivalate, t-amyl peroxypivalate, diisononanoyl peroxide, decanoyl peroxide, succinic acid peroxide, di(n-propyl) peroxydicarbonate, di(sec-butyl) peroxydicarbonate, di(2-ethylhexyl) peroxydicarbonate, t-butylperoxyneodecanoate, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, t-amylperoxyneodecanoate, dimethyl 2,2xe2x80x2-azobisisobutyrate and combinations thereof.
As a preferred embodiment, the initiator is selected from the group consisting of 2,2xe2x80x2-azobis(2,4-dimethylpentanenitrile), 2,2xe2x80x2-azobis(2-methylpropanenitrile), 2,2xe2x80x2-azobis(2-methylbutanenitrile), 1,1xe2x80x2-azobis(cyclohexanecarbonitrile), t-butyl peroxy-2-ethylhexanoate, t-butyl peroxypivalate, t-amyl peroxypivalate, and combinations thereof.
The amount of initiator is any amount that accomplishes the desired end result. However, as a preferred embodiment, said initiator is present to about three mole percent based upon the total moles of all of said monomers I, II, and said copolymerizable monomers.
The polymerization conditions are any temperature and pressure that will produce the desired end result. In general, the temperatures are from about 30xc2x0 C. to about 100xc2x0 C., preferably from about 40xc2x0 C. to about 100xc2x0 C., and most preferably from about 45xc2x0 C. to about 90xc2x0 C. The pressure may be atmospheric, sub-atmospheric or super-atmospheric. The polymerization time is not critical, but generally will take place over a period of at least one minute in order to produce a polymer of corresponding composition.
In step (b), in a transesterification, the polymer step (a) is subjected to said transesterification conditions in said alcohol solvent in the presence of a catalytic amount of a transesterification catalyst. The catalyst is such that it will not substantially react with said alkyl acrylate monomer II, or with said co-polymerizable monomers. The catalyst is selected from the group consisting of ammonia, lithium methoxide, lithium ethoxide, lithium isopropoxide, sodium methoxide, sodium ethoxide, sodium isopropoxide, potassium methoxide, potassium ethoxide, potassium isopropoxide, cesium methoxide, cesium ethoxide, cesium isopropoxide, and combinations thereof, wherein the carboxylic alkoxide anion is similar to the carboxylic alcohol solvent. It is also understood that the catalyst can be alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, cesium hydroxide and combinations thereof. If the monomer being used is xe2x80x94OR, where it is xe2x80x94OR5 (Formula III, then the catalyst is a strong acid such as a mineral acid like hydrochloric acid (HCL).
The amount of catalyst employed is from about 0.1 mole percent to about 2 mole percent of monomer I present in the composition of said polymer.
In a preferred embodiment, the catalyst is added in step (b) as a solution in said alcohol solvent.
The temperature in step (b) is such that the transesterified by-product ester formed can be continually removed from the reaction mixture to form the polymer of I, II, and said copolymerizable monomer. Such temperatures can be from about 50xc2x0 C. to about 200xc2x0 C. In a preferred embodiment, the transesterification reaction is carried out at reflux temperature of said alcohol solvent.
In step (c), the cation-exchange resin is preferably a strongly acidic cation exchange resin. An acidic ion exchange resin, such as sulfonated styrene/divinylbenzene cation exchange resin in hydrogen form is preferably utilized in the present process. Suitable acidic exchange resins are available from Rohm and Haas Company, e.g. AMBERLYST 15 acidic ion exchange resin. These Amberlyst resins typically contain as much as 80,000 to 200,000 ppb of sodium and iron. Before being utilized in the process of the invention, the ion exchange resin must be treated with water and then a mineral acid solution to reduce the metal ion level. When purifying the polymer solution, it is important that the ion exchange resin is then rinsed with a solvent that is the same as, or at least compatible with, the polymer solution solvent. The procedure in step (c) may be similar to those procedures disclosed in U.S. Pat. Nos. 5,284,930 and 5,288,850.
In conjunction with steps (a), (b) and (c) above, it is critical that all three steps be conducted on an anhydrous basis, i.e. where the water level is less than about 5000 parts per million (ppm), in order to avoid possible side reactions, and provide a mechanism to provide a convenient and direct route to a resist composition without having to isolate the product and then carry out additional processing steps.
In optional step (d), an isolation may be accomplished by precipitation of said polymer from the reaction medium by adding said reaction medium to a third or non-alcohol solvent for the product as for example one selected from the group consisting of water, hexane, heptane, octane, petroleum ether, and combinations thereof. It is also within the scope of the present invention to conduct an optional step (e) in place of step (d), wherein after step (c) the polymer in the alcoholic solvent is replaced with a photoresist compatible solvent such as those listed in the prior art set forth herein. An example of such a solvent is propylene glycol monomethyl ether acetate(PGMEA); other solvents are well known in the art. In this manner, the resulting polymer in the PGMEA can be directly treated with other chemicals in order to directly form a photoresist composition without having to isolate the polymer from step (c) and then drying it and then redissolving it for further processing.
As an example of the preparation of the terpolymer described above, there is provided a process for the preparation of a polymer of IV, 
an alkyl acrylate monomer having the formula II, 
and an ethylenically unsaturated copolymerizable monomer selected from the group consisting of styrene, 4-methylstyrene, maleic anhydride, dialkyl maleate, dialkyl fumarate and vinyl chloride, wherein alkyl is having 1 to 4 carbon atoms, comprising the steps of:
a) subjecting a monomer of formula V, 
xe2x80x83along with said monomer II, and said copolymerizable monomer to suitable polymerization conditions in a carboxylic alcohol solvent and in the presence of a free radical initiator at suitable temperature for a sufficient period of time to produce a polymer of corresponding composition;
b) subjecting said polymer from step a) to transesterification conditions in said alcohol solvent in the presence of catalytic amounts of a catalyst at the reflux temperature of said alcohol solvent such that the transesterified by-product acetate formed is continuously removed from the reaction mixture to form the polymer of IV, II, and said copolymerizable monomer;
c) contacting said polymer solution in said alcohol solvent from step b) with an cation-exchange resin in hydrogen form to remove said catalyst; and
d) isolating said polymer from step c) by precipitation from said alcohol solvent;
wherein:
i) R3 is either hydrogen or methyl; and
ii) R4 is either isopropyl or tert.-butyl.
In this example, R3 is hydrogen or methyl and R4 is tert-butyl or iso-propyl. The initiator is selected from the group consisting of 2,2xe2x80x2-azobis(2,4-dimethylpentanenitrile), 2,2xe2x80x2-azobis(2-methylpropanenitrile), 2,2xe2x80x2-azobis(2-methylbutanenitrile), 1,1xe2x80x2-azobis(cyclohexanecarbonitrile), t-butyl peroxy-2-ethylhexanoate, t-butyl peroxypivalate, t-amyl peroxypivalate, and combinations thereof. The alcohol solvent is an alcohol having 1 to 4 carbon atoms selected from the group consisting of methanol, ethanol, isopropanol, tert.-butanol, and combinations thereof. A preferred embodiment is where the alcohol solvent is methanol. The catalyst will not substantially react with said alkyl acrylate monomer II or with said copolymerizable monomers and is either alkali metal hydroxide or alkali metal alkoxide, and further is selected from the group consisting of lithium hydroxide, lithium methoxide, lithium ethoxide, lithium isopropoxide, sodium hydroxide, sodium methoxide, sodium ethoxide, sodium isopropoxide, potassium hydroxide, potassium methoxide, potassium ethoxide, potassium isopropoxide, cesium hydroxide, cesium methoxide, cesium ethoxide, cesium isopropoxide, and combinations thereof. As a preferred embodiment, the catalyst is sodium methoxide and is present in an amount of about 0.1 mole percent to about 2 mole percent based upon the atomic weight of sodium and moles of monomer IV present in said polymer. Furthermore, the said catalyst is added in step b) by dissolving it in said alcohol solvent, and the precipitation of said polymer is accomplished by adding said reaction medium to a third or non-alcohol solvent for the product which is selected from the group consisting of water, hexane, heptane, octane, petroleum ether, and combinations thereof. The preferred third or non-alcohol solvent is water. Thus in this example of the novel process, said polymer is a terpolymer of monomer IV, monomer II, and styrene.
In another facet of the present invention there is provided a process for the preparation of a polymer of IV, 
an alkyl acrylate monomer having the formula II, 
comprising the steps of:
a) subjecting monomer of formula V, 
xe2x80x83and said monomer II to suitable polymerization conditions in an alcohol solvent and in the presence of a free radical initiator at suitable temperature for a sufficient period of time to produce a polymer of corresponding composition;
b) subjecting said polymer from step a) to transesterification conditions in said alcohol solvent in the presence of catalytic amounts of catalyst at reflux temperature of said alcohol solvent such that the transesterified by-product acetate formed is continuously removed from the reaction mixture to form the polymer of IV and II;
c) passing said polymer solution in said alcohol solvent from step b) through an ion-exchange bed to remove said catalyst; and
d) isolating said polymer from step c) by precipitation from said alcohol solvent;
wherein:
i) R3 is either hydrogen or methyl; and
ii) R4 is either isopropyl or tert.-butyl.
In this process for the preparation of a copolymer, the initiator is selected from the group consisting of 2,2xe2x80x2-azobis(2,4-dimethylpentanenitrile), 2,2xe2x80x2-azobis(2-methylpropanenitrile), 2,2xe2x80x2-azobis(2-methylbutanenitrile), 1,1xe2x80x2-azobis(cyclohexanecarbonitrile), t-butyl peroxy-2-ethylhexanoate, t-butyl peroxypivalate, t-amyl peroxypivalate, and combinations thereof. The alcohol solvent is an alcohol having 1 to 4 carbon atoms selected from the group consisting of methanol, ethanol, isopropanol, tert.-butanol, and combinations thereof. The catalyst will not substantially react with said alkyl acrylate monomer II. The catalyst is a member of the group of ammonia, and alkali metal alkoxides selected from the group consisting of lithium methoxide, lithium ethoxide, lithium isopropoxide, sodium methoxide, sodium ethoxide, sodium isopropoxide, potassium methoxide, potassium ethoxide, potassium isopropoxide, cesium methoxide, cesium ethoxide, cesium isopropoxide, and combinations thereof. The catalyst, e.g., can be sodium methoxide and is present in an amount of about 0.1 mole percent to about 2 mole percent based upon the moles of monomer IV present in said polymer.
This invention is further illustrated by the following examples that are provided for illustration purposes and in no way limits the scope of the present invention.